1. Field of the Invention
The present invention relates to an optical fiber laser device provided with a laser active substance inside an optical fiber for receiving an activating light from outside and performing a laser oscillation.
2. Description of the Related Art
In a field of optical communication or optical processing technique, development of an inexpensive laser device with a higher output is requested. It is heretofore known that an optical fiber laser device has a high possibility for satisfying such request.
In the optical fiber laser device, by appropriately selecting a core diameter, a difference in refractive index between a core and a clad and the like, an oscillating mode can be relatively easily made single. Also, by confining light at a high density, interaction of a laser active substance and light can be enhanced. Further, by lengthening the device, a long interacting length can be obtained. Therefore, a spatially high quality of laser beams can be generated at a high efficiency. Consequently, a high quality of laser beams can be obtained relatively inexpensively.
Here, to realize a higher output or higher efficiency of laser beams, an activating light needs to be efficiently introduced to a laser active ion doping region (usually, a core portion) in an optical fiber. However, when the core diameter is set in accordance with wave guide conditions of a single mode, the core diameter is restricted to about dozen of microns or less of the laser active ion doping region (usually, the core portion). It is usually difficult to efficiently introduce the activating light to such a small diameter. To overcome this problem, for example, a so-called double clad type of a fiber laser is proposed.
FIG. 5 is an explanatory view of the double clad type of the fiber laser. As shown in the figure, in the double clad type of the fiber laser, on an outer periphery of a clad portion 16 provided is a second clad portion 17 which is constituted of a transparent substance having a refractive index much lower than that of the clad portion 16. By total reflection resulting from a difference in refractive index between the second clad portion 17 and the clad portion 16, an activating light 13 introduced from an end face is enclosed in the clad portion 16 and a core portion 15. When the enclosed activating light repeatedly passes the doping region of a laser active ion (usually, a core portion 14), the activating light is gradually absorbed by the laser active ion. Thereby, a laser beam with a high output can be obtained (reference document; E. Snitzer, H. Po. Fhakimi, R. Tumminelli, and B. C. McCllum, in Optical Fiber Sensors Vol. 2 of 1988 OSA Technical Digest Series (Optical Society of America, Washington, D.C., 1988), paper PD5.).
In the double clad type of the fiber laser, an inlet port of the activating light can be enlarged as much as dozens to thousands of microns. Therefore, the activating light can be easily introduced to the fiber. Also, a region where laser oscillation takes place can be restricted to dozens of to several microns. Therefore, a light with a laser oscillating wavelength can be advantageously propagated in a single mode, and a highly dense light can be enclosed.
However, the optical fiber laser usually has an disadvantage that a laser oscillating condition is largely varied by an influence of disturbances, for example, oscillation, pressure, sound and the like. This is because in the optical fiber, a laser amplifying medium itself is inseparably formed integrally with a laser resonator itself. Further, a medium has a very large asbestos ratio. The optical fiber is given flexibility, but lacks mechanical strength. Therefore, the influence of disturbances is much enlarged. The positive use of the influence is advantageous in a fiber sensor and the like. Conversely, this is largely disadvantageous for a usual solid laser device. For example, in the optical fiber laser, a transverse mode can be considerably strictly controlled because of clear boundary conditions given by fiber transmission. However, a frequency control or the like relating to a vertical mode of a resonator is difficult. In this respect, it should be said that the optical fiber laser is close to a laser using liquid as a medium, rather than to a solid laser.